Recently, along with the development of high-performance optical amplifiers, an optical amplification repeater transmission system in which a signal light is transmitted a long distance by being amplified has been researched. To realize a further big capacity, a WDM transmission system in which signal light multiplexed of many wavelengths is transmitted through one optical fiber is advantageous. In the optical amplifiers, where the WDM signal light can be wholly amplified, the upgrading such as the increase in number of channels is very easy by using many laser diodes having different wavelength, thereby realizing a low cost.
However, to simply increase the number of channels may cause the shifting of designed levels in the whole system, therefore requiring the readjusting of output power or amplitude ratio of the optical amplifier. This is similarly caused in the case that the number of channels is reduced due to any trouble. To automatically conduct the readjustment, the current number of channels used needs to be supervised.
It is known to analyze the number of channels and optical spectrum in WDM signal light by using a scanning Fabry-Perot interferometer (refer to H.Toba, et al.,"16-channel Optical FDM Distribution/Transmission Experiment utilizing Er.sup.3+ -doped Fiber Amplifier", Electronics Letters, vol.25, No.14, pp.885-887(1989)) In this interferometer, two mirrors are located being opposed in parallel to transmit only a light with a frequency (wavelength) an integer times of the longitudinal mode of a resonator. Varying the position of one mirror by a piezo-element, the interval of the interferometer is repeatedly scanned to analyze an optical spectrum.
Furthermore, it is known to detect the power of a backward amplified spontaneous emission light and thereby control the output of a pumping laser to be constant to make the gain constant regardless of the variation in the number of channels (refer to Y.Nakabayashi, et al., "Er: Doped Fiber Amplifier for WDM Transmission Using Fiber Gain Control", Technical Report of IEICE, OCS94-66, OPE94-89(1994)).
However, in the conventional method, the reliability for long time operation may be reduced since they include mechanical moving elements due to the construction of the interferometer. Further, to scan around 30 nm corresponding to the band of erbium-doped optical fiber amplifier (EDFA), the resonator length of the interferometer needs to be less than 40 .mu.m, therefore making the fabrication difficult.
On the other hand, in the conventional method for controlling output of an optical amplifier, while the gain can be controlled, the number of channels and the shifting of wavelength cannot be supervised.